The present invention relates to holographic recording using a matrix or other configuration, including a geometric configuration, of discrete photorefractive elements. The invention also relates to the use of photorefractive elements and associated matrices for signal processing.
It is well known that bulk photorefractive crystals have been used for applications such as data storage, defect enhancement, optical correlation and convolution, optical interconnects and associative memories. These applications have used mostly commercially available bulk crystals such as the ferroelectrics LiNbO.sub.3 (lithium niobate) and SBN (strontium barium niobate) and the paraelectric BSO (bismuth silicon oxide).
The bulk photorefractive crystals usually are used in a cubic configuration, have a side dimension of about one centimeter and a depth or length of about one-half centimeter. It is difficult to grow larger crystals of good optical quality and, in fact, it is difficult to grow electro-optically superior crystals such as BaTiO.sub.3 (barium titanate) and SBN to even half the stated cross-section. As a consequence of this difficulty in making large bulk crystals, and, additionally as a result of the need for sophisticated, expensive optics to image high resolution patterns (greater than 30 lines per millimeter) onto large surfaces (greater than a few millimeters on a side) without distortion, the inability to selectively control regions of bulk material, and the limitation of certain bulk photorefractive materials to first order nonlinear optical effects (because of the inability to sustain high enough energy densities), wide spread application of photorefractive technology using bulk crystals has not occurred, despite an initial surge of development in the decade of the 1970's.
In the past, composite arrays have been used to simulate bulk materials in imaging applications, e.g., to function equivalently to a single element lens. However, these arrays have been constructed of linear materials such as glass fibers. In addition, the elements of an array formed from such materials cannot be addressed individually. Quite obviously, such materials are not useful for data storage or signal processing.